The present invention relates generally to a control system for controlling a blade carried by a motorgrader used for earthworking and, more particularly, to an improved method and apparatus for controlling the slope of the blade in order to maintain a desired cross slope angle of the surface being worked by the motorgrader.
Control systems for controlling the slope of blades on motorgraders have been utilized in practice in the prior art. For example, a control system is known which employs multiple angle sensors and multiple slope sensors for controlling the slope of a blade on a motorgrader having a two-part articulated frame defined by a rear drive unit and a front steering unit. This blade control system references the orientation of the blade back through the various members of the machine to the rear drive unit. It assumes that the motorgrader is not executing a turn, that the front wheels are not side-tilted and that the blade supporting A-frame is not side-shifted. If one or more of these assumptions is incorrect during operation, the control system will not be able to accurately control the blade slope angle to maintain a desired cross slope. U S. Pat. No. 4,431,060 discloses a further control system for controlling the slope of a blade 30 on a motorgrader including a ground engaging trailing wheel assembly 96. The assembly 96 includes a pitch accelerometer 128 purportedly for sensing the pitch of the blade 30 and a slope accelerometer 130 purportedly for sensing the slope of the blade 30. A trailing wheel 116, which is mounted onto a shaft 110 of the assembly 96, follows behind the blade 30 and remains aligned in the direction of travel of the motorgrader.
The pitch and slope accelerometers 128 and 130 are mounted within a support housing 108 which is rotatably mounted onto the shaft 110. A potentiometer 124 is also mounted to the housing 108 while an adjustable input shaft 126 thereof is secured to a support member 122 which rotates with the blade 30. As the blade 30 is rotated, shaft 110 is rotated by the trailing wheel 116 so that the housing 108 remains in alignment with the direction of travel of the motorgrader. Since the potentiometer 124 remains in alignment with the direction of travel of the motorgrader while its input shaft 126 rotates with the blade 30, the potentiometer is able to sense the degree of rotation of the blade 30. By employing the slope, the pitch, the angle of rotation of the blade and other sensed values, the control system operates to maintain the blade 30 at a desired slope.
This control system is problematic because it employs a ground contact sensor, which includes the trailing wheel 116. When the trailing wheel 116 hits disturbances, such as rocks or clumps of dirt, it is knocked out of alignment from the direction of travel of the motorgrader. As a result, error in the output from the slope and pitch accelerometers 130 and 128 will result since they are mounted to the housing 108, which rotates with the trailing wheel 116. Further, if the trailing wheel 116 looses contact with the ground, such as when the blade is raised, error again will occur in the output from the accelerometers 130 and 128. Finally, due to the environment in which motorgraders are employed, there is a risk that the ground contact trailing wheel assembly might be damaged or torn from the motorgrader while in use.
Therefore, a need exists for an improved blade angle control system capable of measuring the angle of rotation of a blade relative to the direction of movement by a sensor which can be reliably mounted onto a motorgrader without substantial risk of being damaged or torn from the machine. Preferably, the blade angle control system would be capable of accurately measuring the parallel and perpendicular slopes of the blade and control the blade slope without requiring multiple angle sensors as in the prior art.